Some semiconductor devices have a gate structure over a substrate to control conduction between active regions within the substrate. A typical gate structure usually has a layer of polycrystalline silicon (or polysilicon) doped with some type of dopant (impurities) to form a doped polysilicon gate.
The type of the dopant defines the conductivity type of the doped polysilicon gate. An n-type polysilicon gate has a dopant that provides extra electrons. For example, arsenic is usually used as a dopant in an n-type polysilicon gate. A p-type polysilicon gate has a dopant that provides extra holes. For example, boron is commonly used as a dopant in a p-type polysilicon gate.
Some devices have two doped polysilicon gates of different conductivity types placed side by side and sharing the same gate contact that spreads across both gates. When a shared gate contact is used, a dopant from one gate may cross to the shared gate contact and diffuse to the other gate. This is cross diffusion.
The cross diffusion changes the conductivity of the two doped polysilicon gates. A small cross diffuision may cause the device to perform inefficiently. Too much cross diffusion may lead to failure of the device.
Some methods for preventing cross diffusion exist in various forms. Some of these methods, however, either increase the resistance of the doped polysilicon gates or have inadequate prevention of the cross diffusion.